Conventionally, an automotive wiper system typically includes a linkage mechanism for converting the rotational output of an electric motor to a reciprocating sweeping movement of a wiper arm. It is however possible to do away with such a linkage mechanism by controlling the motor so as to alternatingly reverse the rotational direction of the output of the electric motor, and directly transmitting this movement to the wiper arm.
FIG. 1 shows a circuit diagram of a previously proposed prior art windshield wiper system which effects the reciprocating movement of a wiper arm by reversing the rotational direction of an electric motor. The two input terminals of the wiper motor 1 are connected to moveable contacts of first and second relays RL1 and RL2. In each of the relays RL1 and RL2, one of the fixed contacts is connected to a power line, and the remaining fixed contact is grounded. The system further comprises a position detecting plate 5 which includes an electroconductive disk member and turns in synchronism with the wiper motor M to detect the points of reversal of the wiper arm (not shown in the drawings) at the two extreme positions. The position detecting plate 5 includes a continuous circular segment 5a extending over 360 degrees, and a sector segment 5b extending over a limited angle integrally and coaxially with the continuous circular segment 5a. A first contact 6a and a second contact 6b are in sliding engagement with the continuous circular segment 6a and the sector segment 5b, respectively. The first contact 6a is grounded, and the second contact 6b is connected to an input terminal I2 of a controller 4 via an interface circuit.
The sector segment 5b extends over an angle which corresponds to the two points of reversal, and the second contact 6b comes into contact with the sector segment 5b as it reaches each point of reversal. When the second contact 6b is in contact with the sector segment 5b, the two contacts 6a and 6b are electrically connected to each other so that the input level at the input terminal I2 changes from a low ground level to a high power line level. The controller 4 can detect the point of reversal from the level of the signal at the input terminal I2.
When the wiper switch SW is closed and a high signal is thereby applied to an input terminal I1 of the controller 4, electric current is supplied to the motor M in the direction indicated by Ix, and the motor M starts to turn in the direction indicated by X. This rotation continues throughout the time the contact 6b is not in contact with the sector segment 5b. When the motor M has turned until the second contact 6b comes into contact with the sector segment 5b, and the signal resulting from the electric connection between the two contacts 6a and 6b is supplied to the controller 4 to indicate the arrival of the point of reversal, the state of the relays which was as illustrated in FIG. 1 changes to a new state in which the first relay RL1 is switched over, and the two input terminals of the motor M are both grounded. As a result, not only the supply of electric power to the motor M is terminated, but also a regenerative braking action is applied to the motor M because the two input terminals are directly connected to each other. This causes the motor M to come to an immediate stop. The regenerative braking action as used herein means a braking action which is produced in the motor when two input terminals thereof are connected either directly or via a low resistance path.
The controller 4 then issues a signal from an output terminal O2 after a prescribed time delay to switch over the state of the second relay RL2 so that electric current is conducted in the direction as indicated by arrow Iy. As a result, the motor M now turns in the direction indicated by arrow Y, and the sweeping direction of the wiper arm is reversed. Thereafter, this action is repeated until the wiper switch SW is turned off.
According to this system, there is a certain time lag from the time the second contact comes into contact with the sector segment 5b and the time the regenerative braking action of the motor actually starts. This is due to the provision of a CR constant which exists in the circuit associated with the position detecting plate 5. Such a CR constant in the circuit is necessary for ensuring a stable operation of the position detecting plate 5. Also, the operation of the relays involves a certain time delay. The sweeping speed of the wiper arm also changes depending on the presence of oil, dust and other matters on the surface of the windshield. Therefore, some fluctuation is inevitable in the distance by which the wiper blade moves from the point of reversal as defined by the position detecting plate and the point at which the regenerative braking action actually starts. Therefore, the actual terminal position of the wiper blade tends to change depending on the condition of the surface of the windshield. However, this is not commercially acceptable to the user.